Plastic Shrinkage Cracking on a Windy Slab
A 6 in. unreinforced exterior slab on grade is placed at 2:00 p.m. on a day with 15 mph wind, 92 °F air temperature, and 25% relative humidity. The mix has a 0.52 w/c, 4 in. slump, and no synthetic fibers. The finisher floats the surface once about 45 minutes after screeding, then waits for bleed water to disappear before troweling. By 4:30 p.m. the surface shows a network of short, parallel cracks perpendicular to the wind direction, mostly near the slab edges and between control joints spaced at 20 ft. The contractor proposes to fog the surface now and trowel the cracks closed, then cure with a curing compound at 6:00 p.m.
1. Classify the cracks and explain the mechanism, distinguishing them from drying shrinkage and thermal cracking.
Read the worked answer
These are plastic shrinkage cracks. The signature is: short, roughly parallel cracks oriented perpendicular to the prevailing wind, appearing within hours of placement, concentrated at edges and areas of high surface-to-volume exposure, on a hot, dry, windy day.
Mechanism: the evaporation rate from the surface exceeds the rate at which bleed water rises to replace it. The surface layer dries and shrinks while the underlying concrete restrains it, and the tensile stress exceeds the low early-age tensile strength. The ACI 305 evaporation-rate nomogram would put this combination (92 °F, 25% RH, 15 mph wind) well above the ~0.2 lb/ft²/h threshold where precautions are required.
Distinguish from:
- Drying shrinkage: develops over weeks/months, not hours; cracks are typically longer and map-like, often at restrained sections.
- Thermal cracking: from heat of hydration and restraint, usually appears later and in thicker elements; pattern differs.
The timing (within hours), orientation, and weather all point to plastic shrinkage.
2. Evaluate the proposed remedy of fogging and troweling the cracks closed at 4:30 p.m.
Read the worked answer
Fogging now is appropriate and should have been done earlier, but troweling cracks closed at 4:30 p.m. is largely cosmetic and risky. By then the concrete has stiffened; troweling over plastic-shrinkage cracks can trap a weak surface layer, and closing the surface does not heal the crack below. The cracks will likely reappear as the slab continues to shrink.
The real problem is that the mix and practice set this up: 0.52 w/c with no fibers, a single early float, and a long wait for bleed water in conditions where bleed water may never appear (high evaporation). Waiting for bleed water to disappear before troweling is a rule of thumb that fails when evaporation is high.
Better response:
- Fog immediately to stop further evaporation and re-wet the surface.
- Do not trowel cracks closed as a fix; evaluate crack depth and extent with the engineer.
- Begin curing as soon as the surface can take it - the proposed 6:00 p.m. curing compound is too late; curing should start immediately after finishing.
- Document crack width and depth for the engineer's acceptance decision.
3. What should be changed on the next placement to prevent recurrence?
Read the worked answer
Prevention targets the evaporation rate and the mix's bleed capacity:
- Monitor evaporation rate (ACI 305 nomogram) before and during placement; if it exceeds ~0.2 lb/ft²/h, require precautions.
- Use windbreaks and shade; schedule placement earlier or at night; avoid placing in the hottest, windiest part of the day.
- Fog spray or use evaporation retarder immediately after screeding and between finishing operations.
- Consider a mix with higher bleed (less supplementary cementitious replacement of early strength, or a small air content) and synthetic fibers to bridge early cracks.
- Start curing immediately after finishing - curing compound or wet cure - not hours later.
- Add or tighten control joints; 20 ft spacing on a 6 in. slab may be marginal depending on jointing plan, but jointing does not prevent plastic shrinkage cracks.
- Pre-wet the subgrade and forms to reduce moisture loss to the base.